When a trench for a dual damascene structure is formed on an insulation layer made of, for example, SiOC and formed on an underlying layer made of, for example, SiCN, the insulation layer is etched by plasma of a fluorine-based gas such as, for example, C4F8, using a photoresist as a mask to form a via hole on the insulation layer. Also, after an organic material is deposited in the via hole to protect the underlying layer at the bottom of the via hole, the insulation layer is etched by plasma of a fluorine-based gas such as, for example, CF4, using a hard mask for trench formation which is made of, for example, TiN, to form the trench for the dual damascene structure. See, for example, Japanese Patent Laid-Open Publication No. 2007-59666.